It is characteristic of a modern mobile communication system that a mobile station is free to roam and connect from one cell to another within the mobile communication system. If a mobile station does not have an ongoing call, the cell crossover only results in registering to a new cell. If a mobile station MS is handling a call during the cell crossover, the call must also be switched to the new cell by a way which causes as little disturbance to the call as possible. The cell crossover process during an ongoing call is called a handover. A handover may also be carried out within the cell from one traffic channel to another. In order for the mobile communication system to be able to detect a need for a handover and to select a suitable target cell for the handover, various kinds of measurements are required in order to determine the connection quality and field strength levels of the neighbouring cells. A handover from the serving cell to a neighbouring cell can occur, for example, (1), as measurement results of the mobile station/base station indicate a low signal level and/or quality in the present serving cell, and a better signal level can be obtained in a neighbouring cell, or (2), as a neighbouring cell enables communication at lower transmitting power levels. The latter may occur in cases a mobile station is in a border area between cells. In radio networks, the aim is to avoid unnecessarily high power levels and thus interference elsewhere in the network.
The transmitting power of a mobile station is usually controlled from the fixed network by a power control algorithm. The mobile station measures the receive level (field strength) and quality of the downlink signal received from the base station of the serving cell, and the base station of the serving cell, in turn, measures the receive level (field strength) and quality of the uplink signal received from the mobile station. On the basis of these measurement results and the preset power control parameters, the power control algorithm determines an appropriate transmitting power level of which the mobile station is then informed in a power control command. During a call, power control is continuous.
Mobile communication systems of this kind, however, have a problem regarding the post-handover situation. Following the handover, it takes a while before an adequate number of measurement results can be obtained from the mobile station and the base station of the new cell, and before the power control algorithm is able to make an optimal adjustment to the transmitting power levels of the base station and the mobile station. For this reason, the mobile station is first, following the handover, commanded to use the highest transmitting power allowed to it in the new cell. The highest allowed transmitting power ensures an adequate link quality even to those mobile stations that are located in the cell border area as the handover is carried out. If, however, the mobile station is closer by as the handover is carried out, the highest allowed transmitting power might be unnecessarily high. Although the power control rapidly decreases the transmitting power of the mobile station to an appropriate level, the post-handover radio frequency power peaks may cause uplink disturbances in the radio network. Furthermore, overrated power levels unnecessarily shorten the battery life in the battery-powered mobile station.